The deep structure of non–volcanic rifted continental margins

Abstract

Delineating the nature of crustal variations across passive continental margins is fundamental to our understanding of how the uppermost lithosphere deforms under extension. This information is best obtained across margins where the extensional fabric within the crust has not been significantly modified by large-scale syn-rift volcanism. Previous studies of such margins have shown a range of extensional styles, which have been interpreted by various combinations of pure and simple shearing of continental crust and lithosphere. This paper reanalyses several key transects in the North Atlantic, including the conjugate margin pairs of Newfoundland-Iberia, Flemish Cap-Goban Spur and Labrador-W. Greenland. Analysis of both deep multichannel seismic reflection and wide-angle reflection/refraction profiles allows us to produce new joint constructions of velocity and reflectivity depth sections. These sections indicate major asymmetries in the width and faulting style of continental rifting between margin conjugates. They also show a general occurrence of a complex transition zone between extended continental and oceanic crust that is dominated by exposed and serpentinized upper mantle. Because serpentinite has a much lower strength than is typical of continental or oceanic crustal rocks, its existence within the transition zone may have a profound influence on how these margins develop. The possible effects that such time and spatially variable rheologies have on the development of these margins needs to be considered in future geodynamical models.

Margin segmentation of Baffin Bay/Davis Strait, eastern Canada based on seismic reflection and potential field data

“…10) based on Chalmers (1997), Chalmers and Pulvertaft (2001), Chalmers et al (1993 and Louden and Chian (1999). For the northern Labrador Sea, we incorporate the results of Roest and Srivastava (1989), Chalmers (1991), Chalmers and Laursen (1995), Chian and Louden (1992), Chian et al (1995a,b), Louden and Chian (1999) and Funck and Louden (1999).…”

“…Chalmers and Laursen (1995) proposed that seafloor spreading started at chron 27n time. In their interpretation between chron 27n and the edge of the continental shelf, there is a 100-150 km broad region where the continental crust must have been extensively stretched, or serpentinized mantle was unroofed (Chian and Louden, 1994;Chalmers, 1997;Louden and Chian, 1999;Chalmers and Pulvertaft, 2001). In the Labrador Sea, the reconstruction produces a large gap between the two COBs, whereas in the Davis Strait the volcanic rocks meet to produce a continuous igneous province of the Cape Dyer and Disko region basalts.…”

“…1) are interpreted as non-volcanic (Reid, 1994;Keen and Potter, 1995;Todd and Reid, 1989;Reid and Keen, 1990;Srivastava and Roest, 1999;Chian et al, 1995b;Louden and Chian, 1999;Louden and Lau, 2001;Funck et al, 2004b). Volcanic margins are characterized by highvelocity lower crust, SDRs and an associated magnetic anomaly with the high-velocity lower crust (Holbrook and Kelemen, 1993).…”

Models for the evolution of passive margins

“…Previous studies suggested that the reflector corresponds to the BDT, a detachment fault that penetrates the entire lithosphere, an intra-crustal detachment fault, a detachment between the upper crust and the serpentinised mantle. The most probable interpretation, however, (e.g., Louden and Chian, 1999;Reston et al, 1996) is that it marks the transition between a low velocity, highly faulted, upper crust and high velocity serpentinised peridotite.…”

“…Keen et al (1989), for example, argued that while the pattern of faulting at the Flemish Cap, Canada and Goban Spur (Southwest Approaches, UK) conjugate margin pair is asymmetric, the amount of crustal thinning shows a high degree of symmetry. Recently, Louden and Chian (1999) using better data have questioned the degree of symmetry at this margin pair. Moreover, these workers have shown that the Labrador-southwest Greenland conjugate margin pair is initially symmetric, but shows progressively more asymmetry as the locus of rifting shifts to one side of the rift system.…”